The greater diffusion of small particles relative to larger particles can be used to partially separate the species. Diffusion is a process which can easily be neglected at large scales, but rapidly becomes important at the microscale. Due to extremely small inertial forces in such structures, practically all flow in microstructures is laminar. This allows the movement of different layers of fluid and particles next to each other in a channel without any mixing other than diffusion. Moreover, due to the small lateral distances in such channels, diffusion is a powerful tool to separate molecules and small particles according to their diffusion coefficients.
Using tools developed by the semiconductor industry to miniaturize electronics, it is possible to fabricate intricate fluid systems with channel sizes as small as a micron. These devices can be mass-produced inexpensively and are expected to soon be in widespread use for simple analytical tests.
A process called "field-flow fractionation" (FFF) has been used to separate and analyze components of a single input stream in a system not made on the microscale, but having channels small enough to produce laminar flow. Various fields, including concentration gradients, are used to produce a force perpendicular to the direction of flow to cause separation of particles in the input stream (see, e.g. Giddings, J. C., U.S. Pat. No. 4,147,621; Caldwell, K. D. et al., U.S. Pat. No. 5,240,618; Wada, Y., et al., U.S. Pat. No. 5,465,849). None of these references disclose the use of a separate input stream to receive particles diffused from a particle-containing input stream.
A related method for particle fractionation is the "Split Flow Thin Cell" (SPLITT) process (see, e.g., Williams, P. S., et al. (1992), Ind. Eng. Chem. Res. 31:2172-2181; and J. C. Giddings U.S. Pat. No. 5,039,426). These publications disclose channel cells with channels small enough to produce laminar flow, but again only provide for one inlet stream. A further U.S. patent to J. C. Giddings, U.S. Pat. No. 4,737,268, discloses a SPLITT flow cell having two inlet streams. Giddings U.S. Pat. No. 4,894,146 also discloses a SPLITT flow cell having two input streams. All these SPLITT flow methods require the presence of more than one output stream for separating various particle fractions.
None of the foregoing publications describe a channel system capable of analyzing small particles in very small quantities of sample containing larger particles, particularly larger particles capable of affecting the indicator used for the analysis. No devices or methods provide simultaneous measurement of more than one analyte.